The present invention relates to rubber compositions and, in particular, rubber compositions intended for the manufacture of tire casings and elastic bearings for vehicle suspension systems, and particularly to rubber compositions used in the tread of tire casings for trucks, construction and farm vehicles or airplanes.
The compositions of treads for tire casings are generally made by incorporating a reinforcing filler, such as carbon black, in one or more elastomers. In the case of tire casings which must withstand heavy loads or roll at high speed, there is an intense release of heat and the tire casings are subjected to high temperatures. This temperature rise is accompanied by high rolling resistance and fuel consumption and can be the source of damage to the tire. It is therefore desirable to obtain mixtures which generate less heat under these difficult operating conditions in order to avoid premature ageing of the carcass, which must be capable of retread after tread wear. Obtaining such slightly hysteretic mixtures must not, however, be accompanied by deterioration on wear and traction.
By partially or totally replacing the carbon black with silica in a natural rubber-base composition, it is known to the expert that mixtures less hysteretic than those reinforced with carbon black can be obtained and that the use of a coupling agent between the silica and the elastomer makes it possible to limit the interactions between the silica particles which are the source of hysteresis loss (that is, hysteresis) and to improve the reinforcement, in particular, of wear resistance, as described in EP-A-0,501,227.
As silica-elastomer coupling agents of mixtures vulcanizable with sulfur, which are known to the expert, it is possible to mention the alkoxysilanes reactive at one end with silica (silane part) and at the other end with the carbon-carbon double bonds of the diene elastomer, for example, by means of a polysulfide or mercapto group. Such coupling agents can be premixed with silica and possibly grafted on the silica or added during preparation of the mixture, the coupling agents being combined in situ with the silica. Among the coupling agents of this type most used, Si69 can be mentioned, which is the bis-(3-triethoxysilylpropyl) tetrasulfide marketed by Degussa (Rubber Chem. Technol., 1977, Vol. 50, p. 447) or X50S, also marketed by Degussa, which is a 50/50 mixture by weight of Si69 and N330 black.
It is also known to the expert that silanes having an aminoalkyl group are mentioned under the same heading as silanes having methyl or vinyl groups as being not active coupling agents with the diene elastomers (Wagner, Rubber Chem. and Technol., 1976, pp. 703 to 794), for there is no predictable reaction between the amino group and the double bonds of the elastomer. However, in some patents disclosing (i) diene elastomer, (ii) silica and (iii) coupling agent base compositions, it is possible to find cited under the same heading as the sulfo, mercapto or poly-sulfide groups active with the elastomers possessing double bonds, the vinyl, amino and epoxy groups, as, for example, in EP 0,738,613 A1 or U.S. Pat. No. 5,225,011, in which the amino or epoxy alkoxysilanes are described as coupling agents for natural rubber-base compositions possessing as filler a carbon black and silica mix under the same heading as a mercaptoalkoyxysilane and a bisalkoxysilane polysulfide, such as Si69, but obviously not used in the examples given. In fact, it is known to the expert that a vinyl or amino or epoxy group cannot be reactive with the double bonds of a diene elastomer.
However, if an aminoalkyltrialkoxysilane is used, even at very low levels in a composition with majority natural rubber base and a majority silica-type filler, the composition cannot be used, for the scorching mixture (vulcanization lag time almost nil) starts reticulating in the course of the mixing and shaping stages; the use of that mixture, which becomes very hard, and notably its extrudability, is rendered almost impossible on the industrial scale.
On the other hand, the use of alkylalkoxysilanes is known, such as, for example, the hexadecyltrimethoxy and triethoxy silanes, Si 116 and Si216 respectively, marketed by DEGUSSA, free of groups reactive with unsaturated elastomers, but reactive by their alkoxy groups with silica-type fillers, that is, possessing Si--OH groups on the surface, which make it possible to reduce viscosity, improve proccessability and activate the vulcanization of compositions containing one or more polysulfide silanes like Si69 (Technical Information, DEGUSSA AG No. 6000.1 of June 1994). Consequently, an alkyltrialkoxysilane-aminoalkyltrialkoxysilane combination is not therefore envisaged by the expert to increase the almost nil lag time due to the use of aminoalkyltrialkoxysilane.